"Aquaculture" may be generally defined as the provision of favorable conditions for the growth of useful aquatic animals. Aquaculture has been practiced for thousands of years; thus, the first recorded instance thereof was the raising of carp in China in the year 1100 B.C., approximately 3,000 years ago. These fish were raised in ponds, and from that time until comparatively modern times, there was little change in the techniques applied to production, the ponds being generally small and productivity being limited to whatever fish food became available through the natural plant-type growth of each pond. Aquaculture of this general type has been greatly improved in recent years through fertilization of pounds, feeding of pelleted food concentrates, and in general a better understanding through research of fish requirements and problems.
Such "pond" type aquaculture has heretofore been mainly used in the rearing of herbivorous fish, such as tilapia, clarias, milkfish, mullet, and carp, all of which feed to a large extent directly on phytoplankton. It is conventional practice in this type of aquaculture to depend upon natural processes for rejuvenation of the water; i.e., the photosynthetic activity of phytoplankton is relied upon for regenerating nutrients and oxygen.
While herbivorous fish rearing is by far the present largest aggregate commercial aquaculture operation, nevertheless there is a large and increasing demand for a variety of aquatic non-air breathing carnivorous animals, such as several species of salmon, trout, shrimp, spiny lobster, and others. Such carnivorous species are, in general, considered to be much more of a delicacy in most places throughout the world than the herbivorous species, and are therefore highly desirable from a commercial standpoint because of a large demand and generally much higher prices that consumers are willing to pay therefor. Because of such commercial desirability, and because of the relatively low and seasonal availability of such carnivorous species from the fishing industry, there is a rapidly increasing current trend toward the aquaculture of a variety of carnivorous species. To a large extent the prior art endeavors in the aquaculture of carnivorous species have relied upon the aquaculture techniques developed in the pond culturing of herbivorous species, with ration feeding of the carnivorous species. For example, trout farming in the U.S.A. generally involves pellet ration feeding in ponds and private lakes, with natural processes being depended upon for water rejuvenation without any control thereof. Similarly, many Danish and Norwegian trout farms involve trash fish feeding in ponds.
However, such present systems for rearing aquatic nonair breathing carnivorous animals face a continual and major hazard of a "vicious circle" of loss of DO (dissolved oxygen), which may be explained as follows:
These aquatic non-air breathing carnivorous animals obtain their oxygen from the relatively very small concentration thereof which will dissolve in water (approximately 6 to 11 perts per million depending on temperature and salinity). If for any of a variety of reasons, the DO content of the water decreases sustantially, the animals invariably stop or greatly reduce their intake of ration food.
The unconsumed ration food remaining in the rearing system is relatively quickly attacked by aerobic bacteria which are always present and multiply rapidly in the presence of such nutrients. These bacteria then proceed to substantially consume the DO. The DO loss problem is then further and quickly aggravated on the next feeding of ration food to the point where the carnivorous animals being reared either directly die from suffocation, or become weakened and thereby become susceptible to frequently present disease organisms.
Some of the reasons for such an initial drop in DO include the following: (1) crowding of the reared species, particularly in relation to their movement relative to the water; (2) water stagnation, particularly near the bottom where organic matter of all kinds tends to accumulate; (3) water stratification, caused for example by heating of the surface layers by sunlight, particularly with high concentrations of phytoplankton therein which absorb the light, thereby lowering the density of the surface layers relative to lower layers; (4) after bottom layers have become relatively deoxygenated through stratification, then a turnover or mixing of the body of water occurring, as for example by wind action; (5) prolonged decrease in phytoplankton activity (which normally restores DO as well as removing toxic animal wastes), which may occur upon a decrease in sunlight or temperature, or as a result of high Ph levels which cause unavailability of essential carbon dioxide; (6) actual die-off of phytoplankton caused by excessive temperature or sunlight; or (7) leftover food caused simply by overfeeding the carnivorous animals, which may easily occur through (a) a drop in temperature which reduces the digestive rate of such cold-blooded animals and therefore their ability to consume previously satisfactory ration quantities, (b) overestimation of the total weight of live animals in the system, since daily rations must be closely related, as a percentage, to total animal weight in the system, or (c) underestimation of the individual size of the animals in the system, since such animals consume more at smaller sizes relative to their weight.
Substantially all of the above effects are greatly aggravated at points below the euphotic zone (i.e., below the point at which phytoplankton receive sufficient light to be active). Thus, the DO loss hazard is particularly acute in the case of bottom-dwelling species such as shrimp or lobster. It is believed that this problem is a major explanation for the lack of success of several prior art commercial aquaculture ventures attempting to rear American shrimp species.
Accordingly, in such a prior art ration feed rearing system for carnivorous aquatic non-air breathing animals, the inverse relationship between the food remaining in the system and the DO produces what may be considered to be a "vicious circle", with catastrophic results to the animals being reared.
In order to guard against the vicious circle of DO loss, two methods are commonly practiced in the rearing of carnivorous aquatic non-air breathing animals, each of which has serious drawbacks and is often costly. One such method is to conduct the rearing in running water, as for example in net pens, cages, or raceways. This has the serious problems: (1) waste of nutrients, i.e., that large portion not directly converted to animal growth; and (2) pollution of the downstream environment. Second, by the pumping of air to depth into the water so as to directly increase the DO of the water. This has the problems of (1) risking nitrogen supersaturation effects on the animals (many such animals die at nitrogen supersaturations of more than 105 percent of saturation), and (2) while this procedure can restore oxygen to the system, it does not remove waste products, some of which, particularly ammonia which is excreted in large amounts by these carnivourous animals, are highly toxic to the animals.
In addition to the above hazard, there is an important general inverse effect of DO level on growth rate below about 15 PPM DO. Thus, even in running water systems, there is a substantial lowering of DO when practical rations of water to fish are utilized which causes a lessening of achievable growth rates of the fish.